Bitcoin. It makes accepting online payments so easy, you can do it accidentally.

Returning bitcoins to the "from" address doesn't necessarily do what you think it does. If the sender used a hosted (shared) EWallet, the coins that you return will go to someone else other than the sender.You'll probably just want to pick a charity and send them there. If you wanted a charity that accepts Bitcoin, there are many: - http://en.bitcoin.it/wiki/Donation-accepting_organizations_and_projectsThis is the same dilemma that NH State Rep Mark Warden is in. He accepted bitcoin donations and then realized that doing so for campaign contributions might be a probelm because they are sent anonymously, possibly from foreign sources. But he can't return them to the sender. So he'll probably be donating them as well. - http://bitcointalk.org/index.php?topic=102022.0